1. Field of the Invention
The invention concerns a linear actuator for the conversion of a rapid rotational movement with low moment into a slow linear movement with high force. The most various embodiments of this type of control element are known in the mechanical engineering industry, for example as recirculating ball screw, roller screw drive or planetary threaded pinion/drive.
2. Description of the Related Art
Each of the known embodiments has their specific advantages and/or disadvantages, so that the appropriate selection for each respective application is based as a rule upon the most optimal compromise. Certain applications are known, wherein the existing mechanisms do not achieve a satisfactory constructive solution.
At this time there is being developed, under the widely accepted English term "brake by wire", an electrical braking system for automobiles, which will make the hydraulic system used until now obsolete. This hydraulic system is being replaced with electro-motor drive of the brake calipers. A sensor measures how strong the driver steps upon the brake pedal and reports this measurement to a system control microprocessor, which sends a command from the soles of the shoe of the driver to the motors and the brake calipers. All of this functions more rapidly and precisely then with a hydraulic system. In this case the electronic control can incorporate such functions as anti-lock brake system (ABS) or anti-slip control (ASC) and so significantly simplify the complex brakes in automobiles.
In the conventional development of this type of brake system it has unfortunately been shown, that the electrical drive system formed of a combination of drive motor and transmission cannot, with a more, satisfy the task set to it. In particular the inadequate operational effectiveness, for one part, of the high speed reduction and, for the other part, the working principle associated with the transmission, requires electro-motors of high capacity, in order to achieve the necessary pressure of the brake shoes against the brake disk. For the control of this kind of strong motors an upgrading of the conventional electrical on-board voltage from 12 to 60 volts would be inevitable, since even when using large cable diameters the extremely high current would result in corresponding electrical losses in the electrical system. Besides this, large electro-motors would at the same time be heavy, whereby the unsprung mass of the wheel would be increased in an undesirable manner. It must further be taken into consideration that only a small amount of space is available for installation in the wheel well or, as the case may be, in the area of the suspension, so that bulky aggregates cannot be incorporated therein.
There existed thus in particular for the above described application the task for providing a linear actuator of high force and small stroke, which should be capable of converting the electrical energy into mechanical performance with low loss. The task overall has a high degree of difficulty due to the following necessary requirements:
high operational effectiveness PA1 small work space with very high force requirement PA1 sufficiently large work stroke for compensation of wear on the brake disk and linings PA1 compact construction and low weight PA1 resistance to high temperatures and temperature changes PA1 resistance to dirt and water PA1 absolute reliability PA1 vibration stability PA1 ease of service and low service requirement PA1 low production costs